tEchnology scan hEads
➤ which doesn’t produce any noticeable heat affected zone on the material. But in order for them to operate in this way, each pulse has to be separated to avoid putting excess energy into the part. To ablate the material, the laser has to pass over the surface several times, but the pulses always have to be separated. ‘These ultra-short pulsed lasers reach very high
frequencies of several megahertz to get reasonably high power,’ Hofner says. ‘At these frequencies, the scanning optics have to move extremely fast. This is a real challenge for scanner technology to reach these high speeds and maintain the accuracy of the system.’ He adds that ultra-short pulses put high power densities into the optics and specialised fabrication and coating methods are employed for them.
The PSO (position synchronised output)
programming environment developed by Aerotech for the Nmark CLS allows a better registration between successive pulses on each pass, because the laser is pulsing based on the position of the stage. In addition, Rekowski says the scanner using the Nmark CLS controller provides high accuracy and repeatability. ‘Traditional scanners aren’t necessarily that accurate,’ he states.
‘They have thermal drift issues and long term repeatability problems. ‘If you’re working with an ultra-high speed laser and you’re trying to make features on the scale of tens of microns wide then accuracy and repeatability becomes critical,’ he says. ‘We deal with these types of accuracies all the time with our precision linear stages. We’re trying to apply that same positioning capability to a scanner.’
Further increases in
speed will be required to keep up with the pulse rate
According to Rekowski, historically, customers have struggled to get on the order of 10µm accuracy over a field size of 150 x 150mm. With a high-quality set of x-y stages though, that’s not difficult to achieve. The tradeoff with using x-y stages is that they
don’t have the throughput that a scanner would. ‘We want to improve the accuracy of the scanner such that we can get better than 10µm over a 150
x 150mm field of view,’ Rekowski comments. The other advantage of tying the scanner and
servo axes together with a common programming language is the elimination of stitching errors in larger parts. Traditionally, high-precision scanners would use a small field size – of 100 x 100mm, for example – to keep the accuracy high. A part 300 x 300mm would therefore have to be processed in nine 100mm sections. In this instance, stitching errors, when moving from one section to another, can occur due to the angular motion that occurs during the x-y stage motion. Since the scanner and servo are both
controlled directly with the Nmark CLS in the same programming environment, the customer can program in one continuous 300mm x 300mm programming space. ‘The field of view is not fixed to the operational area of the scanner,’ says Rekowski. ‘You don’t need to break a continuous scan into smaller segments according to the field of view of the scanner.’ Speed and accuracy remain important factors in laser scanning devices for high-precision micromachining tasks. And with the emergence of ultrafast lasers further increases in speed will be required to keep up with the pulse rate. l
20 ElEctro optics l DECEMBER 2011/JANUARY 2012
www.electrooptics.com
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